Recently, the Society of Automotive Engineers (SAE) Recommended Practice J2535, Setting Preload in Heavy-Duty Wheel Bearings was reviewed in the interest of eliminating vague phraseology and to provide more specific detail by adding the following definition: Target Preload Setting—the preload force value that is the preliminary level of adjustment from which the final wheel bearing adjustment can be achieved. This is the first instance in having an absolute value of preload force identified that can be used as the basis for recommended preload installation procedures. Now it is possible for spindle nut manufacturers to develop instructions on how their products can be used to install controllable and repeatable preload.
In addition to setting controlled preload, the teachings of the UltraHigh-Integrity Interlocking Nut and Washer System can be used to establish and securely lock the adjusted amount of end play or axial clearance as defined in SAE Recommended Practice J2535. The means to accomplish this are described in the Technology and Maintenance Council (TMC) Recommended Practice RP 618, Wheel Bearing Adjustment Procedures.
At the present time, there are no specific standards or recommendations stating the maximum limit for preload, but from a practical standpoint, the value eventually recommended should be in the same order that provides the measured retardation-which normally exists in wheel-end systems installed with endplay. Wheel bearing engineers have suggested that a range of 2 to 5 pounds of preload force would be acceptable, but they also state that the spindle nut manufacturers are responsible for establishing the specific maximum amount of allowable preload force. Conjecture about this will be resolved following action by the TMC to provide a revised recommended practice describing how to secure an acceptable amount of preload after the reaching the 50 foot pound spindle nut torque level prior to initiating back-off rotation described in the TMC Recommended Practice 618.
U.S. Pat. Nos. 5,795,037 and 7,000,995 disclose systems and methods used to install precise levels of preload on tapered roller bearings of the type used in conventional wheel-end systems of highway trailers. However, the principles described therein only apply to double-jam nut systems. With the new guidelines now established within the tractor-trailer industry, it will now be possible for all spindle nut manufacturers to issue new instructions for installing controlled preload to their existing single nut systems.